A parameter study for modeling MgII h and k emission during solar flares

Abstract: Solar flares show highly unusual spectra, in which the thermodynamic conditions of the solar atmosphere are encoded. Current models are unable to fully reproduce the spectroscopic flare observations, especially the single-peaked spectral profiles of the MgII h and k lines. We aim at understanding the formation of the chromospheric and optically thick MgII h and k lines in flares through radiative transfer calculations. We take a flare atmosphere obtained from a simulation with the radiative hydrodynamic code RADYN as input for a radiative transfer modeling with the RH code. By iteratively changing this model atmosphere and varying thermodynamic parameters, such as temperature, electron density, and velocities, we study their effects on the emergent intensity spectra. We can reproduce the typical single-peaked MgII h and k flare spectral shape and their approximate intensity ratios to the subordinate MgII lines by either increasing densities, temperatures or velocities at the line core formation height range. Additionally, by combining unresolved up- and downflows up to ~250 km/s within one resolution element, we also reproduce the widely broadened line wings. While we cannot unambiguously determine which mechanism dominates in flares, future modeling efforts should investigate unresolved components, additional heat dissipation, larger velocities, and higher densities, and combine the analysis of multiple spectral lines.